Define std::is_callable and std::is_nothrow_callable

Define std::is_callable and std::is_nothrow_callable

	* doc/xml/manual/status_cxx2017.xml: Update status table.
	* include/std/functional (__inv_unwrap): Move to <type_traits>.
	(__invoke_impl): Remove exception specifications.
	(__invoke, invoke): Add exception specifications using
	__is_nothrow_callable.
	* include/std/type_traits (__inv_unwrap): Move from <functional>.
	(__is_callable_impl, __call_is_nt, __call_is_nothrow): New helpers.
	(__is_callable, __is_nothrow_callable): New traits.
	(is_callable, is_callable_v): New C++17 traits.
	(is_nothrow_callable, is_nothrow_callable_v): Likewise.
	* testsuite/20_util/is_callable/requirements/
	explicit_instantiation.cc: New test.
	* testsuite/20_util/is_callable/requirements/
	explicit_instantiation_ext.cc: New test.
	* testsuite/20_util/is_callable/requirements/typedefs.cc: New test.
	* testsuite/20_util/is_callable/requirements/typedefs_ext.cc: New
	test.
	* testsuite/20_util/is_callable/value.cc: New test.
	* testsuite/20_util/is_callable/value_ext.cc: New test.
	* testsuite/20_util/is_nothrow_callable/requirements/
	explicit_instantiation.cc: New test.
	* testsuite/20_util/is_nothrow_callable/requirements/
	explicit_instantiation_ext.cc: New test.
	* testsuite/20_util/is_nothrow_callable/requirements/typedefs.cc:
	New test.
	* testsuite/20_util/is_nothrow_callable/requirements/typedefs_ext.cc:
	New test.
	* testsuite/20_util/is_nothrow_callable/value.cc: New test.
	* testsuite/20_util/is_nothrow_callable/value_ext.cc: New test.

From-SVN: r239145